1. Field of the Invention
The present invention relates generally to sootblower devices which are used to project a stream of fluid cleaning medium against internal surfaces of large scale combustion devices for cleaning the internal surfaces. More particularly, the present invention concerns a control system for such devices.
2. Background Information
Sootblowers are used to project a stream of a fluid cleaning medium such as water, air or steam against internal heat transfer surfaces of a combustion device such as a large scale boiler. The fluid cleaning medium is used to dislodge various combustion by-products, including slag and ash, which become deposited on the heat transfer surfaces. The cleaning medium dislodges the encrustations through thermal and mechanical shock which fractures the encrustations, breaking them free and dislodging them from the heat transfer surfaces. If the encrustations are not removed, boiler efficiency significantly decreases. However, through effective and consistent sootblowing, the efficiency of the boiler can be maintained.
Various types of sootblowers are presently used. One general category of sootblowers is known as the long retracting type. These devices have a retractable lance tube which is periodically advanced into and withdrawn from the combustion device, and may be simultaneously rotated such that one or more nozzles on the lance tube project a jet of cleaning medium which traces a desired cleaning path.
In typical retracting sootblowers, a feed tube is held stationary relative to the sootblower frame which sets outside the combustion device. One end of the feed tube is supplied with the cleaning medium through a poppet valve. The lance tube slidably over-fits the feed tube and its longitudinal sliding and rotational motion is controlled by a transmission gear set contained in a carriage which moves along tracks on the sootblower frame.
The cleaning medium supplied to the feed tube pressurizes the hollow interior of the lance tube. The cleaning medium escapes from the lance tube through one or more nozzles which direct the spray against the surfaces to be cleaned. At the conclusion of the cleaning cycle, the lance tube is retracted and withdrawn from the combustion device to avoid continued exposure to the intense heat and/or corrosive atmosphere inside the boiler which would degrade or destroy the lance tube.
Typically, limit switches are used to determine when the lance tube is fully extended and/or fully retracted. The limit switches are placed at strategic locations along the sootblower frame and are actuated by the carriage moving past them. One limit switch is used to determine when the lance tube is fully extended and signals the drive motor controller to reverse the carriage drive direction so as to retract the lance tube. Another limit switch determines when the lance tube is fully retracted and signals the drive motor controller to turn off the motor at the conclusion of a cleaning cycle.
One concern with the above-described sootblower design is that the limit switches are expensive and often cause maintenance problems, particularly in high temperature and highly corrosive applications such as recovery boilers. Thus, there is a need for a relatively inexpensive and easily maintained sensing and control mechanism for determining when a sootblower lance tube is fully extended and/or fully retracted. Another problem with the above-described sootblower design is that sootblowers in general, and limit switches in particular, require a substantial amount of additional wiring and circuitry for implementation and control. Thus, there is also a need for a means for reducing the amount of additional wiring and circuitry needed to implement and control a sootblower.
Although other types of position sensors could conceivably be used, such as light interrupter or variable reluctance transducers, their use may be limited by the severe operating environment of a sootblower, and can impose cost burdens.